Attenuation mechanisms without a polarization change relative to the source may present benefits that are frequently lost through the use of conventional attenuation schemes. However, with the advent of multi-line lasers and supercontinuum lasers, broadband attenuation has generally become relatively difficult in cases where it is desirous to maintain laser source characteristics through attenuation. The problem may further increase in complexity if true continuous wave (CW) conditions are desirous for a system, e.g., where an attenuator does not have pulse width modulation or similar modulation mechanisms.
An existing method of broadband attenuation includes a circularly variable neutral density filter (CVND). In this case, a transparent optical substrate (e.g., germanium, silicon, some form of sapphire such as synthetic sapphire, or the like) is coated on one side with a metallic Inconel® coating such that the density varies with angle, e.g., coated with a thin film anti-reflection coating and a non-uniform coating for providing attenuation. Unfortunately, the Inconel® coating generally does not provide even attenuation over a large spectrum of interest (e.g., about 1.9 μm-5.0 μm) and the relative spectral content of the laser may change dynamically with CVND angular position. Thus, at least three problems may arise from the current state of the art of CVND technology: (1) multiple reflections; (2) spectrally-dependent attenuation; and (3) polarization-sensitive attenuation. Additionally, CVNDs may require motors with significant weight, moderate size, and very large, heavy cabling to provide power and feedback signals.
Other methods of dynamic attenuation may include the following: a half waveplate positioned between two fixed linear polarizers; one fixed and one rotating linear polarizer; digital micromirror displays (DMDs); source power modulation; angularly varied thin film interference coatings. In the case of DMDs, two methods of attenuation to a laser beam may be provided: pulse width modulation (PWM) and binary modulation. However, PWM may introduce an undesirable frequency element into a CW system, which can ruin fidelity in some cases. Similarly, source modulation through PWM is typically not an option. For binary modulation of a DMD for attenuation, pixels may be turned on and off completely such that attenuation would be provided spatially in a laser beam, but this creates undesirable spatial artifacts in the system and increases calibration complexity.
Thin film-based attenuators may be used, but many prove inflexible for system changes such as laser source changes with different polarization states. An additional disadvantage of this technology may include the heavy motors and associated control cabling often required to rotate the attenuators at a relatively high angular velocity and acceleration in high-speed applications.
Attenuators created from combinations of linear polarizers (one fixed and one rotating) may similarly place requirements on a rotating mount for providing the specified angular velocity and acceleration. Also, the insertion losses from two polarizers may be significant in such a mechanism, and there may be a demand for relatively high extinction ratio polarizers that can withstand high-energy laser irradiance. Further, for a generic laser, the polarization state may be changed using such an attenuation mechanism.
While others have appeared to develop a broadband liquid crystal polarization grating that provides achromatized operation (see, e.g., U.S. Pat. Nos. 8,305,523; 8,358,400; 8,537,310; 8,610,853; 8,982,313; and 9,195,092 to Escuti, et al., where each of the foregoing patents is hereby incorporated by reference), these techniques generally require an intensive fabrication process and, in some instances, require adjustments to the fabrication device to complete fabrication. Also, extremely high sensitivity would be needed for the alignment of multiple grating levels in such prior art systems.
At least because the aforementioned existing techniques require sub-micron alignment, because the existing techniques lack gratings with specifically-designed thicknesses and switchability to obtain a desired overall attenuation, and because the existing techniques generally do not maintain a polarization state of a laser beam passing through the gratings, there remains a need for improved devices, systems, and methods for attenuation.